Each byte consists of 10 bits, usually. There is a start bit, a stop bit and 8 data bits. That can vary as the numbers of those bits can vary, but let's leave it at 10 bits for now.
The RCREG buffer is 2 bytes deep. However, the Receiver Shift Register (RSR) can't really be counted because it might be almost full when you check if there is data, so the first byte could arrive the moment after you check it.
That leaves the 2 byte buffer. So the time is 2 * 10 / 19200 = 1.04 ms